Valve announced yesterday that five free-to-play MMOs would arrive on Steam this week. Steam already carries a number of MMOs, but this is the first time free-to-play (F2P) titles will be available in the digital download store. Spiral Knights, Forsaken Worlds, Champions Online: Free for All, Global Agenda: Free Agent, and Alliance of Valiant Arms (AVA) will be the launch titles, all of which are available now.

All of the games may be available, but each of the five launch titles will host in-game events and specials to celebrate the Steam launch. Each game will also offer special in-game content to players who try the “F2P game of the day” on Steam, starting with Spiral Knights on Wednesday the 15th and concluding with AVA on Sunday the 19th. You can find all of the games in Steam’s new F2P category.

Gamers have been able to buy and download traditional subscription-based MMOs for years now through Steam, but this is the first time free-to-play games have appeared. The move is partially in response to the growing F2P industry, and also in response to other digital download stores that offer F2P games in their catalogs.

Since free-to-play MMOs rely on micro-transactions for revenue, Valve has made the same back-end purchasing system used by Steam for game sales, DLC sales, and other premium content available to F2P game publishers. This means that gamers will be able to buy upgraded armor, skills, or any other premium offering in their favorite F2P MMO the same way they buy hats and weapons in Team Fortress 2, or a map pack for their favorite first-person shooter.

These five launch titles won’t be the last free-to-play games to land in Steam. Valve has indicated that additional titles are on the way, and that they’re more than welcome in the Steam catalog.